Silence cgroups v1 read-only filesystem warning

Avoid warning messages when booting systemd-nspawn containers and using
hybrid or legacy cgroups. systemd-nspawn mounts the cgroups v1 controller
tree as read-only so these errors are expected and not problematic.
Partially fixes #17862.

Test plan:

- Before: `mkosi --default .mkosi/mkosi.fedora boot`

```
‣ Processing default...
Spawning container image on /home/daan/projects/systemd/image.raw.
Press ^] three times within 1s to kill container.
systemd 247 running in system mode. (+PAM +AUDIT +SELINUX -APPARMOR +IMA +SMACK +SECCOMP +GCRYPT +GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP +LIBFDISK +PCRE2 +PWQUALITY +P11KIT +QRENCODE +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +XKBCOMMON +UTMP +SYSVINIT default-hierarchy=unified)
Detected virtualization systemd-nspawn.
Detected architecture x86-64.

Welcome to Fedora 33 (Thirty Three)!

Queued start job for default target Graphical Interface.
-.slice: Failed to migrate controller cgroups from , ignoring: Read-only file system
system.slice: Failed to delete controller cgroups /system.slice, ignoring: Read-only file system
[  OK  ] Created slice system-getty.slice.
[  OK  ] Created slice system-modprobe.slice.
user.slice: Failed to delete controller cgroups /user.slice, ignoring: Read-only file system
[  OK  ] Created slice User and Session Slice.
[  OK  ] Started Dispatch Password Requests to Console Directory Watch.
[  OK  ] Started Forward Password Requests to Wall Directory Watch.
[  OK  ] Reached target Local Encrypted Volumes.
[  OK  ] Reached target Paths.
[  OK  ] Reached target Remote File Systems.
[  OK  ] Reached target Slices.
[  OK  ] Reached target Swap.
[  OK  ] Listening on Process Core Dump Socket.
[  OK  ] Listening on initctl Compatibility Named Pipe.
[  OK  ] Listening on Journal Socket (/dev/log).
[  OK  ] Listening on Journal Socket.
[  OK  ] Listening on User Database Manager Socket.
dev-hugepages.mount: Failed to delete controller cgroups /dev-hugepages.mount, ignoring: Read-only file system
         Mounting Huge Pages File System...
sys-fs-fuse-connections.mount: Failed to delete controller cgroups /sys-fs-fuse-connections.mount, ignoring: Read-only file system
         Mounting FUSE Control File System...
         Starting Journal Service...
         Starting Remount Root and Kernel File Systems...
system.slice: Failed to delete controller cgroups /system.slice, ignoring: Read-only file system
```

After: `mkosi --default .mkosi/mkosi.fedora boot`

```
‣ Processing default...
Spawning container image on /home/daan/projects/systemd/mkosi.output/image.raw.
Press ^] three times within 1s to kill container.
systemd 247 running in system mode. (+PAM +AUDIT +SELINUX -APPARMOR +IMA +SMACK +SECCOMP +GCRYPT +GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP +LIBFDISK +PCRE2 +PWQUALITY +P11KIT +QRENCODE +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +XKBCOMMON +UTMP +SYSVINIT default-hierarchy=unified)
Detected virtualization systemd-nspawn.
Detected architecture x86-64.

Welcome to Fedora 33 (Thirty Three)!

Queued start job for default target Graphical Interface.
[  OK  ] Created slice system-getty.slice.
[  OK  ] Created slice system-modprobe.slice.
[  OK  ] Created slice User and Session Slice.
[  OK  ] Started Dispatch Password Requests to Console Directory Watch.
[  OK  ] Started Forward Password Requests to Wall Directory Watch.
[  OK  ] Reached target Local Encrypted Volumes.
[  OK  ] Reached target Paths.
[  OK  ] Reached target Remote File Systems.
[  OK  ] Reached target Slices.
[  OK  ] Reached target Swap.
[  OK  ] Listening on Process Core Dump Socket.
[  OK  ] Listening on initctl Compatibility Named Pipe.
[  OK  ] Listening on Journal Socket (/dev/log).
[  OK  ] Listening on Journal Socket.
[  OK  ] Listening on User Database Manager Socket.
         Mounting Huge Pages File System...
         Mounting FUSE Control File System...
         Starting Journal Service...
         Starting Remount Root and Kernel File Systems...
[  OK  ] Mounted Huge Pages File System.
[  OK  ] Mounted FUSE Control File System.
[  OK  ] Finished Remount Root and Kernel File Systems.
         Starting Create Static Device Nodes in /dev...
[  OK  ] Finished Create Static Device Nodes in /dev.
[  OK  ] Reached target Local File Systems (Pre).
[  OK  ] Reached target Local File Systems.
         Starting Restore /run/initramfs on shutdown...
[  OK  ] Finished Restore /run/initramfs on shutdown.
[  OK  ] Started Journal Service.
         Starting Flush Journal to Persistent Storage...
[  OK  ] Finished Flush Journal to Persistent Storage.
         Starting Create Volatile Files and Directories...
[  OK  ] Finished Create Volatile Files and Directories.
         Starting Network Name Resolution...
         Starting Update UTMP about System Boot/Shutdown...
[  OK  ] Finished Update UTMP about System Boot/Shutdown.
[  OK  ] Reached target System Initialization.
[  OK  ] Started Daily Cleanup of Temporary Directories.
[  OK  ] Reached target Timers.
[  OK  ] Listening on D-Bus System Message Bus Socket.
[  OK  ] Reached target Sockets.
[  OK  ] Reached target Basic System.
         Starting Home Area Manager...
         Starting User Login Management...
         Starting Permit User Sessions...
[  OK  ] Finished Permit User Sessions.
[  OK  ] Started Console Getty.
[  OK  ] Reached target Login Prompts.
         Starting D-Bus System Message Bus...
[  OK  ] Started D-Bus System Message Bus.
[  OK  ] Started Home Area Manager.
[  OK  ] Started User Login Management.
[  OK  ] Reached target Multi-User System.
[  OK  ] Reached target Graphical Interface.
         Starting Update UTMP about System Runlevel Changes...
[  OK  ] Finished Update UTMP about System Runlevel Changes.
[  OK  ] Started Network Name Resolution.
[  OK  ] Reached target Host and Network Name Lookups.

Fedora 33 (Thirty Three) (built from systemd tree)
Kernel 5.9.11-arch2-1 on an x86_64 (console)
```